What is the Vertical cavity surface-emitting laser (VCSEL) diodes?
VCSEL laser diode (TO-Can package)
VCSEL diode structure
The vertical-cavity laser diode or VCSEL is a type of laser diode that emits laser light perpendicular to the active layer and the top surface. It is in contrast to the conventional edge-emitting laser diodes, where the laser light output is parallel to the active layer. In a VCSEL diode, the optical cavity is formed by the two distributed Bragg reflectors (DBRs), which are parallel to the wafer surface. VCSEL diodes normally operate from 770 to 950 nm and can deliver output power from mW to hundreds of watts. They are available in both free-space and fiber-coupled type. These diodes are ideal for high-density data transmission, gesture recognition, 3D IR imaging, LIDAR, metro area network, and gigabit ethernet applications.
Understanding quantum well structure
Generally, in a VCSEL diode, the upper reflective mirror is doped as P-type and the lower reflective mirror is doped as N-type. The DBRs have high reflectivity larger than 90 %. The requirement of high reflectivity is due to the small optical cavity as compared to the edge-emitting laser diodes. The VCSEL diode has a narrow (in few micrometers) active region is in between the two DBR reflectors. Mostly, the active region consists of one or more quantum wells for laser light generations. A Quantum well is formed when a layer of the low bandgap substrate material with a thickness range (≈ 5–20 nm) is sandwiched between the two high bandgap substrate materials. Usually, Gallium arsenide (GaAs) is sandwiched between the aluminum gallium arsenide (AlxGa(1-x)As). The short active region with a quantum well structure allows the VCSEL diodes to operate in single-mode with high efficiency. It can also operate in a multi-mode configuration.
Benefits of VCSELs:
1. The VCSEL diodes have a large output aperture as compared to edge-emitting laser diodes. Hence, the VCSEL diodes have a low divergence angle than edge-emitting laser diodes. So, the VCSEL diodes have high coupling efficiency with optical fibers.
2. Lower threshold current requirements than edge-emitting laser diodes because of the short active region.
3. High beam quality with low power.
4. High-density optical data processing only can be done by using the 2-D VCSEL arrays.
5. Much high power can be obtained by VCSEL arrays. A 2-D VCSEL array consists of many thousand emitters with a spacing between them is some tens of microns. It emits power in the range of 10 or 100 watts in continuous-wave operation.
6. On-wafer testing capability solves the quality-related issues during fabrication. But, in the edge-emitter laser diodes, the testing can only be done after the production process; hence, if any error occurs in testing, then the production material and processing time will be wasted.
7. Compatibility with IC processing for integrated optics.
8. No risk of catastrophic optical damage because of lower optical intensity at the facet.
Specifications details of Vertical-cavity surface-emitting laser (VCSEL) diodes:
Central wavelength: Represents the wavelength of laser emitted from the VCSEL diode. The wavelength is represented in nm (nanometre). Mostly, the VCSEL laser diodes can have a wavelength range of 770nm to 950nm. Higher-wavelength range is also available.
Technology: Vertical-cavity surface-emitting laser (VCSEL)
Configuration: Single emitter/ Array. A single emitter VCSEL diode has a single emitter to emit the light. An array type VCSEL diode has an array of emitter on its emitting surface.
Operation mode: CW laser / Pulsed laser
Type: Free space/ Fiber coupled. A free space VCSEL diode emits the laser light in free space, and a fiber coupled VCSEL diode emits light through the fiber optic medium.
Longitudinal modes: Single/ Multi-mode
Spectral width (FWHM): It is the width of the optical spectrum at the half-maximum power. It is represented in nanometre. Usually, the spectral width of the VCSEL diodes can vary in the range of0.65nm to 1.5nm.
Output power: Represents the output power of the laser diode. The VCSEL diodes output power can vary from milli-watts to hundreds of watts range.
Threshold current (Ith): It is the current at which the output optical power increases sharply. When the laser diode is biased below this threshold current, the output optical power will be very low. Usually, the threshold current of the VCSEL diodes can vary from milli-Ampere to Ampere range.
Operating current: It is the driving current or input currents when the laser diode reaches the rated power. Usually, the operating current of the VCSEL diodes can from milli-Ampere to hundreds of Ampere range.
Operating voltage: Represents the supply voltage. Mostly, a VCSEL diode can have the operating voltage range of 1.7V to 2.7V. Other rating is also available.
Reverse voltage: Represents the maximum allowable reverse voltage, when it is applied across the laser diode. Beyond the reverse voltage, the diode will damage. Usually the reverse voltage of a VCSEL diode is 5 V.
Beam Divergence Angle (θ): Represents the beam divergence angle of the laser light. It is represented in degrees. Usually, the VCSEL diodes have beam divergence angle in the range of 10˚-25˚.
Operating temperature: Represents the safe operating temperature of the VCSEL diode. Typically, it has a range of -20 °C to +85°C.
Storage temperature: Represents the storage temperature range of VCSEL diode. Typically, it has a range of -40 °C to +85°C.
Package type: Represents the type package used in the VCSEL diodes. Mostly, the VCSEL laser diodes are available in the Chip on sub-mount, sub-module, module, Butterfly (BTF), TO-Can, Surface-mount, C-mount, bar, chip, CE-mount, and HHL module packages.
